Horse pacing in the field

Question
I have a 5 year old ex racer mare.  She is pretty chilled out lady who is turned out with our other gelding 24/7 on a large pasture, both horses get on well however the gelding seems the more dominant horse. The mare is of reasonable weight but isn't a very good doer and so the problem I have discovered worries me. At night time, as soon as the light drops, she paces the field, doing laps of the field over and over again. She doesn't seem particularly stressed just walks at quite a pace, there is now a track where she walks. She has plenty of grass and hay is put out for the horses, im worrying however that all this walking and no eating will stop any condition and cause problems such as ulcers and colic etc..

Is there any reason for this behaviour and is there anything I can do to stop it? (stabling ism an option)

Thank you
Immy

Answer
this sounds like a stall vice that is occurs during stress, the horse is finding comfort from moving, I would NOT stall, the horse may have poor night vision and get worried.

Putting another horse may help since the herd would be busy working out pecking orders and other issues, you can also place obstacles on the path to make walking the path hard.

Barrels, logs, hose, plastic trash bags filled with something, tires, anything to make the horse change the routine of the path, when it become more work to pace the horse should take the easy way and stop pacing.

I have lots of videos on stall vices and discuss it on my site.